In recent years, there have so far been proposed a wide variety of systems designed to estimate the direction of the visual line of the driver on the basis of an image of a region about a driver's face taken by a vehicle-mounted camera mounted in a vehicle, and detect an inattentive driving state of the driver on the basis of the estimated direction of the visual line to alert the driver. These systems are needed to estimate the direction of the visual line with high accuracy.
The visual line estimating apparatuses fall into two broad categories. In one category, the visual line estimating apparatus is designed to estimate the visual line on the basis of an image of a region zoomed in an eye. In other category, the visual line estimating apparatus is designed to estimate the visual line on the basis of an image of the region including the whole face taken in wide angle.
The visual line estimating apparatus designed to estimate the visual line on the basis of the image of the region zoomed in the eye is capable of estimating the visual line with high accuracy, but has the drawback of failing to estimate the visual line at the time of moving a head. Therefore, the visual line estimating apparatus is not appropriate to estimate the visual line of the driver.
In contrast, the visual line estimating apparatus designed to estimate the visual line on the basis of the image of the whole face taken in wide angle is capable of estimating the visual line even if the head is moved relatively free. Therefore, the visual line estimating apparatus is appropriate to estimate the visual line of the driver. However, the visual line estimating apparatus has a characteristic that the accuracy of estimating the visual line is tended to be low by the reason that the image corresponding to an eye region has low number of pixels and low resolution.
In general, the visual line estimating apparatus designed to estimate the visual line on the basis of the image of the whole face taken in wide angle is adapted to estimate the direction of the human visual line by detecting the center of a human pupil in the taken image. In this case, false detection of the pupil frequently occurs at a low luminance part about the eye region by the reason of the low resolution of the eye region.
This results in the fact that many significant errors by the false detection of the pupil are included in the visual line measuring results. This leads to the fact that the visual line measuring results should not be directly outputted for estimating the direction of the visual line with high accuracy. Accordingly, the visual line measuring results are needed to be smoothed to remove the influence of the errors.
Therefore, techniques for smoothing the visual line measuring result with some kind of signal processing technology have been conventionally provided. For example, the conventional technology performing a smoothing operation with using wavelet analysis to obtain the visual line estimating result with high accuracy is provided (see, for example, Patent Literature 1).